I really have fallen in love with drawing inks. They are so beautiful to use. I have explained how they can be used in a more experimental way in my Experimenting with Inks post.
There are different types of water based inks available, but my personal favourite are Seawhite Drawing Inks. The colours are bright and vivid, thanks to their high pigment content.
I have found that Seawhite Drawing Inks react better to bleach and salt compared to other water soluble inks on the market. This is because they are not waterproof or permanent.
Unfortunately not being permanent means the colours will fade if exposed to sunlight or UV light, but they will not fade under incandescent light. They will also not fade if they are covered, for instance when they are in a sketch/drawing pad or in a portfolio. If you want to frame your work, there is special glass available, such as UV absorbing glass, plexiglass or you can use acetate. The special glass will protect your work and ensure its colours don't fade over time.
If you need waterproof inks, which are necessary if you want to layer colours on top of each other, then I would recommend Winsor and Newton drawing inks. They contain shellac, which is a binder, making them waterproof and permanent. Bleach and salt don't react very well to these inks and the inks dry very fast, which I personally found made it slightly difficult to work with. It really depends on how you work and what effects you want to create.
You can purchase Seawhite Drawing Inks on-line from Artesaver. Winsor and Newton Inks are easily obtainable from most art shops and on-line.
I have done all the pieces here using Seawhite inks. They also produce a white ink which creates some lovely effects. This can be seen clearly in the pictures below.